Devices for determining particle size are now well known, and it is also well known that lasers can be, and have been, heretofore utilized to achieve particle size measurements (see, for example, U.S. Pat. No. 3,406,289 to Schleusener). In addition, particle size measurement utilizing an open cavity laser is shown and described in my U.S. Pat. No. 4,571,079, and by a passive cavity in my U.S. patent application Ser. No. 552,689, filed Nov. 17, 1983, issued June 10, 1986 as U.S. Pat. No. 4,594,715.
Refinements in extinction particle size measurement utilizing open cavity laser devices have also been heretofore described by R. G. Knollenberg and B. Schuster in "Detection and Sizing of Small Particles in Open Cavity Gas Lasers", Applied Optics, Volume 11, No. 7, November, 1972, pages 1515-1520.
Submicron particle sizing devices utilizing light scattering in an open cavity laser device has also been heretofore described by R. G. Knollenberg in "An Active Scattering Aerosol Spectrometer", Atmospheric Technology, Number 2, June, 1973, pages 80-81. Refinements have been described by R. G. Knollenberg in "Active Scattering Aerosol Spectrometry", National Bureau of Standards Special Publication 412, issued October, 1974, pages 57-64; by R. G. Knollenberg and R. E. Luehr in "Open Cavity Laser `Active` Scattering Particle Spectrometry from 0.05 to 5 Microns", Fine Particles, Aerosol, Generation measurement, Sampling and Analysis, Editor Benjamin Y. H. Liu, Academic Press, May, 1975, pages 669-696; by R. G. Knollenberg in "Three New Instruments for Cloud Physics Measurements: The 2-D Spectrometer, the Forward Scattering Spectrometer Probe, and the Active Scattering Aerosol Spectrometer", American Meterological Society, International Conference on Cloud Physics, July, 1976, pages 554-561; by R. G. Knollenberg in "The Use of Low Power Lasers in Particle Size Spectrometry", Proceedings of the Society of Photo-Optical Instrumentation Engineers: Practical Applications of Low Power Lasers, Volume 92, August, 1976, pages 137-152; by R. G. Knollenberg in "`In Situ` Optical Particle Size Measurements in Liquid Media" presented at Semiconductor Purewater Conference, Palo Alto, Ca., Jan. 13-14, 1983; and by R. G. Knollenberg in "The Measurement of Particle Sizes Below 0.1 Micrometers", Journal of Environment Science, January-February, 1985.
A linear array of detectors has also heretofore been utilized in conjunction with parallel processing of the electrical signals generated by each detector to achieve data acquisition (see, for example, my U.S. Pat. No. 3,941,982).
Known particle measuring devices have been heretofore utilized for a variety of purposes, including determining the presence and/or size of particles in various gases, including air. With particular respect to airborne particles, tolerance limitations and the effects of particulate contamination from the environment has made it necessary to utilize effective contamination control in order to enable fabrication of many of the devices now in use. In particular, precision manufacturing, such as is required, for example, for microelectronic systems, has largely been made possible by the development and application of clean rooms and clean devices.
For many years, standard clean rooms of Class 100 or Class 1000 were more than adequate for essentially all of the electronic devices in use. However, when the present generation of micro-computers came into use, the need for micro-electronic components, such as large capacity memory chips, has resulted in the development of devices that are extremely susceptible to contamination during manufacturing.
The effects of particulate contamination during fabrication of such devices is that the yield of usable product is greatly reduced. The contamination particles can, for example, interfere with lithographic imaging integrity, or they can result in either open or short circuits, and poisoned domains, depending on their nature. At this time, microchip manufacture appears to be the most critical operation in the electronics manufacturing industry that is affected by particulate contamination.
The semiconductor VLSI (Very Large Scale Integrated Circuit) industry has continued to push the state-of-the-art in air particle counters used to certify clean rooms. The need for much higher standards reflects the demands of the VLSI circuit manufacturer as well as filtration improvements that achieve much lower levels of contamination.
Most now known aerosol counters have one cubic foot per minute (1 cfm) sample flow rates. However, to achieve reasonable statistics in a Class 1 environment, it is necessary to sample many cubic feet of air if sensitivity is limited of 0.5 microns (.mu.m). Since the particle population increases as size decreases, most air particle counter manufacturers have therefore chosen to size much smaller particles to more readily develop the appropriate statistical base.
For example, at 0.1 .mu.m, the average particle size distribution found in a clean room would provide nearly 100 times as many particle counts &gt;0.1 .mu.m compared to counts &gt;0.5 .mu.m. Thus, the more sensitive the particle counter the less time is required for room standard certification.
In addition, the manufacturers are producing devices with geometries including features that are smaller than 0.5 .mu.m. Thus, in addition to generating a statistical base in the shortest period of time, higher sensitivity provides known particle size information on more potential defective generators.
With the advent of lasers, the ability to size particles via light scattering as small as 0.1 .mu.m became a routine practice since lasers can have all of their energy focused to a small dimension of high intensity. Several devices now on the market provide 0.1 .mu.m sensitivity but none of these devices are capable of sampling at a 1 cfm flow rate, and, in fact, are generally capable of no more than a 0.1 cfm flow rate at 0.1 .mu.m sensitivity.
Semiconductor manufacturers also require high purity gases with low particulate microcontaminants for a variety of processes. It is necessary to make measurements at line pressures (up to 150 P.S.I.) in most cases. Some of these gases are also high molecular weight gases which scatter more light than air (i.e., a mix of largely oxygen and nitrogen) and thus molecular scattering must be reduced even if flow rates less than 1cfm are adequate.
Thus, the potential statistical advantage at smaller particle size is now partially lost in high molecular scattering environments. Clearly a combination which achieves high sensitivity in such an environment (including, for example, achieving a high flow rate of up to or exceeding 1 cfm) at high sensitivity (to detect particles having a size at least as small as 0.1 .mu.m) is needed.